Phenylalanine-ammonia-lyase, P.A.L. (E.C. 4.3.1.5) was purified from white light-incubated potato tuber discs, and antibodies against the purified P.A.L., raised in rabbits. Immunological techniques, based on the ability of the anti-potato P.A.L. serum to form immunocomplexes specifically with P.A.L., were used to measure P.A.L. protein levels in plant tissue extracts. Plants in which P.A.L.-activity appears to be under photocontrol were investigated. In potato tuber discs, the changes in measurable P.A.L.-activity were accompanied by changes in the amount of immunologically detectable P.A.L. protein; however, there was no apparent increase in P.A.L. protein for the phytochrome-mediated increase in P.A.L.-activity in mustard cotyledons. P.A.L. levels in gherkin hypocotyls and tobacco cell suspension cultures were also investigated. The results were discussed in terms of the roles of de novo enzyme synthesis, activation/inactivation and allosteric mechanisms, in the photocontrol of P.A.L. levels in these plants. Antibodies were also raised against a commercially available, purified preparation of ascorbic acid oxidase, A.A.O. (E.C. 1.10.3.3), from Cucurbita sp. A radioimmunoassay was developed to measure A.A.O. levels in plant tissue extracts. Changes in A.A.O.-activity in germinating pumpkin seeds, in the presence and absence of white light, were followed, and the amounts of A.A.O. protein determined. Activation or modification mechanisms and de novo enzyme synthesis were involved in the increase in A.A.O.-activity. A.A.O. levels in mustard cotyledons were also investigated. The results indicate that the photocontrol of enzyme levels operates via several mechanisms, either acting together or in a co-ordinated manner, within each plant; allowing the activity of specific enzymes to be modulated by a range of environmental factors, including light, wounding or infection and level of nutrients, and also by the developmental stage of the plant.